Cork is bark from the cork oak, e.g. Quercus suber L, which grows predominately in countries near the Mediterranean Sea. Wine corks are produced from reproduction cork, which is bark that has re-grown after the original bark has been stripped off the tree.
The production of cork stoppers commences with the stripping of the reproduction bark from the tree to provide cork slabs. These slabs are stored for up to two years. Usually, the slabs are then graded and often bundled, boiled in water and stacked once more. Abundant mould growth may occur during storage.
The further production steps vary according to the actual cork stopper type, the intended use thereof, and any special demands the end-user might have.
Several types of cork stoppers are known.
Natural cork stoppers are inter alia used as closures for wine bottles. Such stoppers are cut or punched generally in the cork slab longitudinal plane.
Natural cork slices or disks are used inter alia in the production of laminate cork stoppers typically used as closures for champagne bottles. Still further, cork slices are used for sealing purposes at the inner bottom of various screw caps. Slices are usually cut perpendicular to the longitudinal direction of the cork slab.
Around 50% of the slab material can be used for natural cork stoppers or slices. The remaining material, viz. cork pieces or crumbs of varying size, is generally used for preparing granulate cork. Very small cork particles can be used directly in the heating system of the factory as an energy source.
Granulate cork can be used for various purposes, among other things in the production of agglomerated stoppers. To this end the crumbs are ground, cleaned and classified into various particle ranges. Binders, and if desired other additives such as plasticizers and cutting aids, are mixed with the granulated cork of the desired particle size, and the composition is moulded or extruded into shape. Optionally, the thus formed product is then polished.
Laminate cork stoppers are usually used for large stoppers such as champagne bottles. Such stoppers comprise a stopper body and one or more slices or disks of natural cork at the lowermost body part. The body part is typically made of agglomerated cork. The different portions are typically glued together.
Liqueur or liquor stoppers usually comprise a cork body part, be it natural or agglomerated, and a top part. The top part is typically made from plastic, wood or cork. The two portions are glued together.
The manufacturing process of cork slices or cork stoppers, whether natural, agglomerated or laminated, usually includes a step of bleaching the cork. The traditional bleaching process uses hypochlorite, usually calcium hypochlorite. Oxalate is then used as a reductant. The precipitation of calcium oxalate endows the stoppers with an appealing white appearance. Alternatively, hydrogen peroxide is used, with citric acid or the enzyme catalase for the subsequent degradation of residual hydrogen peroxide.
This bleaching is for the purpose of cosmetic appearance and disinfection.
Finally, the water content of the cork articles is adjusted to the desired level, usually 5-8%. This is for the purpose of ensuring the microbial stability of the stoppers. Usually, the stoppers are dried in a stream of warm air, and then packaged.
Cork is a highly desirable stopper material, inter alia due to its excellent gasketing characteristics and its high resistance to water, most organic liquids and all but strong acid and alkali solutions. However, frequently a characteristic cork taint is imparted to packaged goods in contact with cork, be it beverages, foods or various perfumed compounds. Cork taint is an off-flavor (taste or odour foreign to the product), frequently described as musty, mouldy or earthy. Astringency, bitterness and tannic flavor are specific variants of cork off-flavor.